1. Field of the Invention
This invention relates to a process for containing solid wastes. More particularly, this invention relates to a novel process for containing, compacting, and storing solidified radioactive waste including a mixture of crystalline phase of a zeolite with an absorbed metal chloride salt and a glass phase.
2. Description of Related Art
Hazardous wastes are produced by a number of industrial processes, such as chemical, biological, and nuclear processes. The wastes may be in the form of gases, liquids, or solids. The proper disposal of hazardous waste is of considerable importance to industry, due to environmental concerns and regulations. The final disposal of hazardous waste is typically in a landfill or a geological repository.
Prior to final disposal, the hazardous waste may be chemically or physically modified to improve the stability of the material for long term storage at a disposal site. Chemical preparation can take the form of neutralization, wherein the waste is converted into a nonhazardous form prior to disposal in a landfill or the like, or prior to use in another industrial process. Further, the hazardous waste may also be converted into a more stable chemical form. Typically, the hazardous waste is concentrated, where possible, to reduce the volume of the waste prior to disposal. In the case of hazardous waste in the form of gases and liquids, the waste may be absorbed or converted into a semi-solid or a solid form for ease of containment and to reduce potential problems associated with uncontrolled movement or redistribution of the hazardous waste. The hazardous waste is then placed in a landfill. In cases when the waste is particularly toxic, the waste may be placed within a secondary containment vessel which is in turn placed in a landfill or disposal site. Secondary containment of hazardous waste is of particular importance as it relates to radioactive waste, as the handling and transport of the waste in its uncontained form presents significant health and safety problems.
The development of processes and systems for placement of hazardous wastes into a secondary containment vessel and subsequent storage of the contained waste presents significant problems. In particular, the development of a packaging process is difficult when the hazardous waste remains toxic for an extended period of time, as is the case with radioactive waste. The problems presented by hazardous waste that remains toxic for an extended period of time are twofold; first, the hazards to which the process operators are exposed, and second, the long-term stability of the storage system must be considered. Since the hazardous waste may be toxic to human beings, even upon brief exposure, the containment loading system must be such that it can be operated by individuals wearing protective clothing. In the case of radioactive waste, which may prove fatal to individuals exposed even briefly thereto, the process should be capable of being controlled from a remote site. Second, in the case of radioactive waste, which may remain toxic for hundreds of years, the long term durability of the containment system is of great importance. The secondary containment system should be such that it is stable under a wide variety of environmental conditions, such as, temperature variations, moisture, and chemical reaction with compounds normally present in the environment.
One process for the containment of spent nuclear fuel involves the electrometallurgical treatment of the fuel, a process which generates a chloride salt of the hazardous waste. The chloride salt is then immobilized within a two-phase composite ceramic that is referred to as a glass-zeolite composite material. The zeolite component of this composite incorporates the waste salt into its crystalline lattice or cage structure. The glass-zeolite material resembles dense polished marble.
Consolidation and secondary containment of the glass-zeolite material can be achieved by the hot isostatic pressing (hereinafter referred to as "HIP") process. In the HIP process, a mixture of glass-zeolite material is loaded into a stainless steel canister by means of uniaxial cold-pressing techniques. Layers of blended powders are successively pressed into the canister until the container is full. Uniaxial pressing is used to compact the powder within the canister to an initial or green density of approximately 40% solids. After the blended powder is loaded into the canister, a cover is welded onto the canister using appropriate welding techniques. The canister cover incorporates a tube used for pre-HIP processing evacuation of the canister.
Next, the loaded canister is then heated to approximately 500.degree. C. (775.degree. K) under a vacuum for approximately 16 hours, to assure the removal of trapped moisture and gases. The canister is then cooled to room temperature. Once cooled, the evacuation tube is crimped and sheared off while the canister is maintained under a vacuum. The vacuum tight crimp may also be welded to insure the tube remains sealed during further HIP processing. The canister is then heated to 1200.degree. C. at pressures up to 175 MPa (1,727 atm) and maintained at that temperature for one to two hours. The canister is then cooled to room temperature. The entire process of chamber evacuation, heat-up, high temperature soaking, and cooling takes place over a period of 20 to 24 hours.
The HIP process, although providing a potential process for consolidating and containing hazardous waste, has a number of deficiencies. The HIP process is designed as a batch process, wherein a single batch of hazardous waste can be sent through the process at one time. The HIP process is difficult to redesign as a continuous process. The process cycle, the period from when the canister is loaded, through degassing, heating and cooling involves a considerable amount of time (approximately one day). It would be desirable to design a process that would provide for continuous operation or semicontinuous operation and a shorter cycle time.
An additional consideration, as with any industrial process, is the level of maintenance necessary to keep the process equipment operational. The quantity of maintenance and the time required to perform the maintenance is dependent on a number of factors; the operating conditions of the process, the physical size of the processing equipment, the amount of material to be processed, and the complexity of the processing equipment. Processing problems arise due to the operating conditions of the HIP process--extreme high temperatures and pressures. The operating pressure during the HIP process cycle, from 1500 to 2000 atmospheres, places a significant amount of mechanical stress on the process equipment and the containment vessel. The operating temperature for the HIP process, on the order of 1200.degree. C., places additional stresses on the process equipment. Due to these extreme operating conditions, maintenance on the equipment is a high priority.
The operating equipment associated with a pilot plant evaluation of the HIP process occupies several cubic meters for equipment to process hazardous waste samples on the order of one to two kilograms. The scale-up of the process equipment to handle commercial size batches of hazardous waste on the order of 25 to 50 kilograms may occupy a space of 10 to 20 cubic meters. Maintenance on the processing equipment of this size would require a significant amount of time due to the size of the equipment. The maintenance period is further increased when the hazardous waste is radioactive thereby requiring special safety precautions during the maintenance cycle.
The last factor, the complexity of the processing equipment, adds significantly to the maintenance cycle in the case of the HIP process. The HIP process requires the use of high efficiency vacuum pumps, high pressure pumps, high temperature heating elements, in addition to sophisticated handling and transport equipment for moving the HIP canister into and out of the processing area. Further, specialized robotic tools may be necessary to weld and crimp and seal the HIP canister. The amount of time required to maintain any of these pieces of process equipment is significant. When these components are brought together in one process the length of time needed to maintain all the components become significant.
The HIP processing time combined with a safe maintenance schedule makes the HIP process unattractive for handling large amounts of hazardous material that is associated with a typical commercial operation.
Thus, the need exists for an efficient process for the consolidation and compaction of solid or semi-solid hazardous waste which provides for the continuous consolidation and compaction of hazardous waste. Preferably, the desired process should require a minimum number of processing steps and be operable at conditions close to standard temperature and pressure. Further, the process should be such that it can be easily controlled and monitored from a remote location. In addition, the process should utilize simple processing equipment that can be easily maintained.